Hello A.,
Regressing back to baby ways for 3 year olds is very common. They are battling between being the big kid & wanting the "stress free life" of their baby years. You will often see regression behavior when there is stress of some sort or they are feeling overwhelmed.
Take a moment and look at her life in the past week or so. Has there been a stress, extra expection of her, a change in her day? If not, talk with her and listen to her. It may be a stress you cannot see and she just needs to vent, because even at 3, kids needs to vent.
I hope this helps.
R. Magby